1. Field of the Invention
The present invention relates to an optical network, more particularly to an optical network employing a WDM (wavelength-division-multiplexing) technique.
2. Description of the Related Art
As a WDM technique enables the transmission of a plurality of optical signals through a single strand of an optical fiber, it has become possible to transmit a plurality of very high-speed mass-storage optical signals. With the aid of a switch that add/drop the optical signal in the optical layer, it is possible to build an optical network based on the WDM technique.
A WDM optical network is generally classified into a ring network using an optical add/drop multiplexer and a mesh network using an optical cross-connect. The WDM optical network transmits very high-speed mass-storage data through each optical fiber, thus requiring the WDM optical network to effectively cope with any undesirable failures in the system. In case of the mesh network, nodes of each optical network are connected to each other through a plurality of optical fibers, so that a protection switching is carried out through a complicated procedure at a low-speed when the system fails. However, in case of the ring network, only two or four strands of optical fibers are connected to the optical add/drop multiplexer, thereby enabling easier switching when the system fails. For this reason, the ring network is widely used in the market.
A node of a WDM ring network includes an optical add/drop multiplexer having a switching unit for adding/dropping an optical signal and a switching device for performing a protection switching of the network. The WDM ring network is divided into a path protection switching network and a link protection switching network depending on the protection switching manner. The WDM ring network uses two or four strands of optical fibers. In addition, the WDM ring networks are classified into a unidirectional network and a bidirectional network depending on the transmission direction of data. Particularly, a conventional WDM ring network including two strands of optical fibers and bidirectionally transmitting the optical signal uses a link protection switching method utilizing a loop-back of the optical signals.
FIGS. 1A and 1B are respectively illustrates a construction and a protection switching according to a conventional bidirectional optical network using a link protection switching. As shown in FIG. 1A, each node of a ring network includes optical add/drop multiplexers 10a to 40a and 10b to 40b for adding/dropping optical signals through inner and outer rings 2 and 4, and 2×2 switching devices 110 to 180 for executing a switching for the protection purposes. The outer ring 4 transmits optical signals having wavelengths of λ1, λ2, λ3, . . . λN, and the inner ring 2 transmits optical signals having wavelengths of λN+1, λN+2, λN+3, . . . λ2N. The outer ring 4 transmits the optical signals clockwise and the inner ring 2 transmits the optical signals counterclockwise.
When an error occurs in an optical fiber link, the optical network transmits an optical signal to an opposite direction by looping-back the optical signal through two 2×2 switching devices, which are positioned at the both ends of the link with the error. For example, referring to FIG. 1B, if an error occurs in an optical link connecting an optical add/drop multiplexer 10a to an optical add/drop multiplexer 20a, the optical signals having wavelengths of λ1, λ2, λ3, . . . λN intended to be transmitted to the optical add/drop multiplexer 20a from the optical add/drop multiplexer 10a are looped back to an optical add/drop multiplexer 10b through a switching device 120 and transmitted counterclockwise through the inner ring 2. The optical signals having wavelengths of λ1, λ2, λ3, . . . λN transmitted through the inner ring 2 are transferred to the optical add/drop multiplexer 20a from an optical add/drop multiplexer 20b through a switching device 130, thereby completing the switching.
During the normal operation mode of the ring network, 2×2 switching devices 110 to 180 are maintained in a bar state, so that a signal applied to an input part i1 is transferred to an output part o1, and a signal applied to an input part i2 is transferred to an output part o2. However, if an error occurs in the ring network, 2×2 switching devices 110 to 180 maintain in a cross state, so that a signal applied to the input part i1 is transferred to the output part o2. As shown in FIG. 1B, when a switching device 130 is positioned in the cross state, in addition to the optical signals passing through the link with the error, the optical signals having wavelengths of λN+1, λN+2, λN+3, . . . λ2N, which are transmitted in a counterclockwise direction from the optical add/drop multiplexer 20b to the optical add/drop multiplexer 10b, are looped back so that the optical signals are transmitted in a clockwise direction through the outer ring 4. Then, the optical signals are transferred to the optical add/drop multiplexer 10b from the optical add/drop multiplexer 10a through the switching device 120. Switching devices that are remote from the link having the fault are still maintained in the bar state.
However, two drawbacks are created if a bidirectional self-healing ring network including two strands of optical fibers is established using the above-mentioned conventional technique. First, it is necessary to at least double the processing capacity of a multiplexer and a demultiplexer consisting the optical add/drop multiplexer for real application as the conventional technique requires to simultaneously multiplex and demultiplex the loop-back signals as well as the transmitted optical signals in order to perform the recovery process.
FIG. 2 shows a construction of a conventional optical cross-connect. On the assumption that upper optical add/drop multiplexers are operated as the nodes of the outer ring 4 and lower optical add/drop multiplexers are operated as nodes of the inner ring 2, the optical signals transmitted through the upper multiplexers have wavelengths of λ1, λ2, λ3, . . . λN, and the optical signals transmitted through the lower multiplexers have wavelengths of λN+1, λN+2, λN+3, . . . λ2N. However, multiplexing and demultiplexing operatons have to be carried out for an upper optical add/drop multiplexer 10a. That is, the capacity of a multiplexer 13 and a demultiplexer 12 has to be determined, by considering the loop-back of the transmitted optical signals in such a manner that the optical signals having wavelengths of λN+1, λN+2, λN+3, . . . λ2N can pass through the upper optical add/drop multiplexer 10a. In the same way, the multiplexing and demultiplexing works have to be carried out for a lower optical add/drop multiplexer 10b to allow the optical signals having wavelengths of λ1, λ2, λ3, . . . λN to pass through the upper optical add/drop multiplexer 10a. Accordingly, the capacity of the multiplexer and the demultiplexer of the optical add/drop multiplexer for transferring N signals is determined as 1×2N.
Another drawback is that the optical signal passing through a normal link is looped back together with the optical signal passing through the link where the error occurred. Referring back to FIG. 1, optical signals having wavelengths of λN+1, λN+2, λN+3, . . . λ2N and passing through the normal link in the inner ring 2 are looped back, so they are transmitted in a clockwise direction of the outer ring 4. As a result, the transmitted optical signals are interrupted and suffers data loss.
Therefore, there is a need for an improved bidirectional WDM ring network that is cost effective to implement and requires less overload.